The present invention relates to producing an electronic device, such as an organic light-emitting diode (OLED), having an electrode with enhanced injection properties. Further, the present invention relates to an electronic device comprising an electrode with enhanced injection properties.
Organic light emitting diodes (OLEDs) represent a promising technology for display applications or illumination applications. A typical light-emitting diode includes a substrate. On the substrate there is arranged a first electrode, which usually acts as a hole-injecting anode. On the anode there is provided a luminescent region comprising an organic electroluminescent material. At last, a second electrode, which is usually used as an electron-injecting cathode, is deposited on the organic electroluminescent material.
When a voltage is applied across the first and the second electrodes, electrons are injected from the cathode into the luminescent region and holes are injected from the anode to the luminescent region. The electrons and holes recombine in the luminescent region, so that light is emitted.
The known diodes could be subdivided into top emitting diodes and bottom emitting diodes. In top emitting diodes the generated light is reflected by the first electrode and emits through the transparent or substantially transparent second electrode. In bottom emitting diodes the generated light emits through the transparent or substantially transparent first electrode/substrate and is reflected by the second electrode.
In contrast to the conventional components being based on inorganic materials the advantage of the known devices containing organic materials consists in that it is possible to provide large-surface elements, like displays or screens. Further, due to their low process temperature in contrast to inorganic materials, these materials could be easily provided on flexible substrates, thereby being usable in a new set of applications for displays and screens.
However, compared to the light-emitting diodes based on inorganic materials, organic light-emitting diodes are operated with a higher operating voltage. This originates inter-alia from the fact that the injection of charge carriers, i.e. electrons or holes, from the electrodes into the organic layer is less efficient, i.e. the common organic light-emitting diodes do not have electrodes which exhibit ohmic or barrier-less injection.
In order to overcome the above-mentioned problem, i.e. to enhance the injection properties and reduce the operating voltage, WO 02/41414 A1 proposes to provide an organic light-emitting diode comprising the following features. First, there are provided two electrodes, i.e. a hole-injecting anode and an electron-injecting cathode. In contact to the electrodes there are provided layers of transport materials, i.e. a hole-transporting material (HTM) on the anode and an electron-transporting material (ETM) on the cathode. The HTM-layer and the ETM-layer are doped, i.e. the HTM-layer is p-doped (holes) and the ETM-layer is n-doped (electrons). On the HTM-layer and the ETM-layer, respectively, there are further provided blocking layers between which the emitter material (EM), which emits the light during recombination, is arranged.
Though the above-described arrangement is able to enhance the injection properties and to reduce the operating voltage, the enhancement of the injection properties and the reduction of the operating voltage are still not sufficient. Further, the lifetime of the known organic light-emitting diode is decreased, i.e. the diode is subjected to an accelerated ageing, which leads to a decrease of the luminosity of the emitter material (EM). Thus, already after a short running time the light emission efficiency is low.